Mammalian ras p2l proteins like other C-proteins bInd GDP and GTP. It is assumed that a growth signal stimulates the conversion of p2l-GDP to p2l-GTP at the plasma membrane and that active p2l-GTP interacts with an effector molecule to transmit an internal signal. However, it has not been clarified how inactive GDP-bound p2l is reactivated. Studies were carried out to search for the factor(s) regulating this particular step in p2l recycling. We have identified a novel membrane factor, which markedly stimulated the guanine nucleotide exchange activity of ras proteins. The ras guanine nucleotide exchange factor (rGEF) was purified from bovine brain to near homogeneity by 4-step column chromatographies. The purified RGEF exhibited a single major 35kd protein. rGEF increased the exchange rate of GDP in normal and oncogenic ras proteins to 30 - 40 fold. Since the factor was free from GDP/GTP binding activity and non-specific GDP hydrolysis activity, we hypothesize that rGEF may regulate GDP/GTP exchange reaction of ras proteins in response to external signals. To further characterize this factor, we are currently trying to clone the gene encoding rGEF. These findings are extremely interesting, implying that the biological activity of ras proteins may be regulated by a guanine nucleotide exchange system in a manner similar to translation elongation factor EF-Tu or the G-proteins of adenylate cyclase system.